1. Field of the Invention
The subject matter of the present invention is a method of detecting a light signal and a corresponding lidar system.
2. Description of the Prior Art
Lidar detection systems essentially comprise means, such as a laser, for emitting a light signal toward an object to be studied and a device for receiving a signal returned from the object by diffusion or reflection, the means and the device being synchronized with each other. Certain characteristics of the object can be deduced by processing the returned signal. The problem addressed here is that of supplying a signal of sufficient intensity to be read by the processing means. In some prior art lidar detection systems, a counting method is used in which photons are individually converted into electrons in a detector, with a known multiplication factor sufficient to distinguish the signal from the background noise of the system. However, the method is limited to very weak signals. Another method consists in passing the signal resulting from converting photons into electrical charges through a current amplifier preceding the processing means. As the background noise of the system is subject to the same amplification, this method is suitable only for high signal values. In practice, a plurality of detection systems that are calibrated differently must be used when the successive sample values of the signal have different intensities.
The patents EP 0 905 284 and EP 1 111 908 describe a charge accumulation and coupling (CCD) photosensor that comprises an array of cells arranged in rows and columns. The light signal is first collected over an image region and converted into electrical charges that are displaced periodically through a memory region to a read device.
The sensor has the benefit of lending itself to convenient sampling of the signal on the memory rows; however, it does not solve the problem of excess noise when reading samples of low intensity. In the above patents, in addition to integrating charges on the last row of the image region, samples corresponding to successive firings are integrated in the memory region before reading. It is therefore no longer possible to process a particular firing and good synchronization of firings is required. If a portion of the signal is skewed, all of the samples are affected by the error and are lost. Cell saturation is also a risk for the high samples that are accumulated.
The invention is concerned with improving lidar systems by using CCD photosensors that differ from those used in this application in the prior art in that they amplify the signal under conditions that are simultaneously reliable and less constraining than the accumulation of a plurality of firings.
The signal is amplified in the read region instead of in the memory region and by means of a charge multiplication process such that the individual results of the firings can be considered.